The present invention relates to vision testing equipment and more specifically to instruments and methods for use in conducting optometric examinations.
As the use of video display terminals has become more widespread, so too have certain ophthalmological afflictions associated with their utilization. The alphanumeric characters displayed on video screens are made up of dots or pixels which do not have well defined borders and therefore are difficult for the eye to focus upon. Further, since video screens are maintained at a constant distance of about 50 cm from the user's eyes, the same eye muscles are in constant use in focusing on the screens. These factors cause significant amounts of stress and fatigue on the eyes of video display terminal users which are often aggravated by the fact that many such users utilize their equipment for extended periods on a day after day basis. The stress associated with video display use frequently results in peculiar types of eye problems requiring special corrective prescriptions in the spectacles selected for the users afflicted with these problems.
In order to accurately diagnose these problems, appropriate test equipment and test procedures must be provided. In accordance with the process currently used by medical practitioners to determine the spectacle requirements of typical patients, an apparatus is placed in front of the eyes of the patient which enables the doctor to rapidly change a wide selection of lenses while the patient views a set of test images through the lens changing apparatus. As the patient focuses on the test images, the doctor assesses the status of the muscles inside the patient's eyes and judges their degree of relaxation through the use of a retinoscope. The doctor determines the combination of lenses and the prescription best suited to the patient by changing the lenses until he detects the combination which provides the most relaxed state in the eye muscles of the patient.
As may be understood from the above, the fitting of corrective lenses is basically a trial and error process in which the doctor observes the reaction of the patient's eye muscles to an appropriate test image for different combinations of lenses. However, without a test image which accurately simulates the conditions under which the patient may experience eye problems, a prescription for suitable corrective lenses may not be reliably determined. When presented with an image which does not have sharply defined edges or which is slightly out of focus, the eye will respond by reverting to a level of tonic muscle activity known as the resting point of accommodation, having a focal length which may be different than the focal length with a sharply defined image placed at the same distance from the eye. Accordingly, the prescription required for eyeglasses used with a video display terminal can differ from a prescription for use in viewing printed material.
Currently available equipment does not provide a satisfactory system for generating test images which simulate the characteristics of video display terminals. Consequently, most medical practitioners have been left with no recourse except to make educated guesses as to the lens corrections which may work best for their patients and to have their patients go back to work and try the new prescription out. This is, however, a time consuming, expensive, inaccurate and generally unsatisfactory method of proceeding to provide spectacle prescriptions.
It is therefore an object of the present invention to provide an improved system for testing the vision of video display terminal users which allows for accurate determination of the best corrective lens prescriptions for such patients.
It is another object of the present invention to provide an improved apparatus which accurately simulates alphanumeric characters as presented on a video display screen and which can be conveniently used in accordance with current optometric test procedures.
It is a further object of the present invention to provide an improved optometric instrument for use in determining prescriptions for corrective lenses which is economical, compact and simple to use.